Nippers and Turkey together form a kind of "clock" that marks the passage of the work day. Turkey is an older man who is an admirable worker in the morning, but, once dinner is done at 12, becomes more and more red-faced and passionate about his copying work -- to the point of making many mistakes:
The difficulty was, he was apt to be altogether too energetic. There was a strange, inflamed, flurried, flighty recklessness of activity about him. He would be incautious in dipping his pen into his inkstand. All his blots upon my documents, were dropped there after twelve o’clock, meridian.
The suggestion is that Turkey is red-faced in the afternoons because he has been drinking. The "red-ink" Nippers says he spends all his money on is probably liquor.
Nippers, on the other hand, is younger, more ambitious, yet prone to indigestion. Nippers, unlike Turkey, isn't a drinker, but his impatience with being a mere scrivener, coupled with his morning indigestion, makes him extremely irritable in the mornings:
Though concerning the self-indulgent habits of Turkey I had my own private surmises, yet touching Nippers I was well persuaded that whatever might be his faults in other respects, he was, at least, a temperate young man. But indeed, nature herself seemed to have been his vintner, and at his birth charged him so thoroughly with an irritable, brandy-like disposition, that all subsequent potations were needless. When I consider how, amid the stillness of my chambers, Nippers would sometimes impatiently rise from his seat, and stooping over his table, seize the whole desk, and move it, and jerk it, with a grim, grinding motion on the floor, as if the table were a perverse voluntary agent, intent on thwarting and vexing him; I plainly perceive that for Nippers, brandy and water were altogether superfluous.
Turkey does his best work in the mornings; Nippers calms down in the afternoons. Between the two of them, the lawyer is able to get a full day's work. It is, as he says, "a good natural arrangement under the circumstances."
Bartleby, of course, is a different story. Whereas Turkey and Nippers obey a sort of natural law, each having their own time for working, Bartleby works (or doesn't) whenever he wishes. His mantra of "I would prefer not to" can be understood in many ways. It is, of course, an outright refusal to work; but it can also be understood as a determination to take life on his own terms, come what may. It may be that ultimately what Bartleby "prefers not" to do is to be trapped in the mundane existence of people like Turkey and Nippers.